You can easily add them by following the instructions on their site.
On immutable fedora it can be done via
curl -o - https://repository.mullvad.net/rpm/stable/mullvad.repo | sudo tee /etc/yum.repos.d/mullvad.repo
rpm-ostree uninstall mullvad-vpn --install mullvad-vpn
# after reboot, if not working
sudo systemctl start mullvad-daemon
Idk for the how but airvpn does for comparable prices. This coming from a fellow multi-tb Linux iso torrenter. Also I assume you mean VPN unless mullvad does VPS stuff I don't know of.
VPS and VPN. Everyone hosting stuff that makes Police known on their doors